4-12-2

Union Pacific 9000-series

The prototype, UP 9000, as preserved at the Los Angeles County Fairplex in Pomona, California.

Type and origin Power type Steam Builder American Locomotive Company Build date 1926–1930 Total produced 88

Specifications Configuration 4-12-2 UIC class 2′F1′ h3 Gauge 4 ft 8 1⁄2 in (1,435 mm) Driver dia. 67 in (1,702 mm) Length 102 ft 7 in (31.27 m) Adhesive weight 355,000 lb (161,025 kg; 161 t) Total weight 782,000 lb (354,709 kg; 355 t) Fuel type Coal Fuel capacity 42,000 lb (19,051 kg; 19 t) Water cap 18,000 US gallons (68,000 l; 15,000 imp gal) Boiler pressure 220 lbf/in2 (1.52 MPa) Cylinders Three Cylinder size Outside (2): 27 in × 32 in (686 mm × 813 mm); Inside (1): 27 in × 31 in (686 mm × 787 mm)

Performance figures Tractive effort 97,664 lbf (434.4 kN) Factor of adh. 3.63

Career Operators Union Pacific Railroad Class UP-1 through UP-5 Numbers 9000–9087 Disposition One preserved, remainder scrapped

Under the Whyte notation for the classification of steam locomotives, 4-12-2 represents the wheel arrangement of four leading wheels, twelve coupled driving wheels, and two trailing wheels.

Other equivalent classifications are:
AAR wheel arrangement: 2-F-1
UIC classification: 2′F1′ (also known as German classification and Italian classification)
French classification: 261
Turkish classification: 69
Swiss classification: 6/9

This arrangement was named the Union Pacific type, after the only railroad to use it.

Only one type of 4-12-2 was built: the Union Pacific Railroad's 9000-series locomotives, 88 of which were built by ALCO between 1926 and 1930. These locomotives were used to increase the speed of freight trains in flat country, and were fairly successful, but were maintenance nightmares, largely because of their use of an inside third cylinder driving the cranked second driving axle between the frames. There was no inside valve gear to worry about, however. ALCO had obtained permission to use the conjugated valve gear invented by Sir Nigel Gresley. This system used two hinged levers connected to the outer cylinder's valves to operate the inner cylinder's valve. The 9000 class locomotives were the largest to use Gresley gear.

Between 1934 and 1940 eight of the first fifteen locos had their Gresley gear removed and were converted to a "double Walschaerts" valve gear which utilized a double eccentric (return) crank and second link on the right side (similar to the gear Baldwin used on its 3-cylinder experimental compound 4-10-2 #60000), which operated the valve for the inside cylinder. Union Pacific referred to this system as the "third link." The 4-12-2's constructed from 1928 utilized roller bearings in the Gresley lever bearings, thus none of these engines were converted. The pre-1928 engines not converted received the roller bearing levers in 1940, and no further conversions were made.